The Soviet invasion of Poland began shortly after the country found itself under attack from Germany on September 1, 1939. Poland was overwhelmed and had been struggling to defend itself against the blitzkrieg tactics employed by the Germans. The Nazi invasion was swift and effective, capturing significant territories within a week. On September 17, the Red Army, following the terms of the Molotov-Ribbentrop Pact signed on August 23, 1939, advanced into Polish territory from the east. This agreement had secret protocols that outlined the division of Eastern Europe, including Poland, between the two totalitarian regimes.

The Soviet troops encountered very little resistance, as the Polish forces were primarily concentrated in the west, fighting the Germans who were advancing rapidly. The Polish government, realizing their defeat against Germany, had already begun to evacuate. As the Soviets moved in, they portrayed their actions as a protective measure for the Ukrainian and Belarusian populations in Poland. The Red Army swiftly occupied Western Ukraine and Eastern Galicia, effectively sealing the fate of Poland and marking a significant escalation in the regional conflict.

The invasion led to the establishment of Soviet control over half of Poland, setting the stage for decades of oppression under Soviet rule. For Poland, this occupation would result in significant social and political upheaval, affecting the nation profoundly in the coming years. Meanwhile, both the Nazis and Soviets would continue to navigate their tactical alliance until Germany would ultimately betray the pact in 1941 with Operation Barbarossa.